UV irradiation is known to induce HIV LTR-directed gene expression in tissue culture systems. Transgenic mice, harboring constructs consisting of the HIV LTR linked to the chloramphenicol acetyl transferase (CAT) gene, were exposed to UV-B or UV-C light sources. Increased CAT expression was detected in ear specimens from irradiated transgenic animals. CAT activity reached a maximum during the 7 hr following irradiation and decreased to basal levels by day 7. In vitro mutagenesis techniques have been employed for investigations of the role of cis-acting HIV LTR elements during productive virus infection. We previously ascertained that removal of both the NF-(kappa)B and Sp-I motifs abolished virus infectivity. In a series of "reconstruction" experiments, NF-(kappa)B and Sp-I elements, either singly or in combination, were added to an "empty" LTR associated with an infectious molecular clone of HIV. Virus stocks were obtained by transfection and tested for infectivity and cytopathicity. Our findings indicate that juxtaposition and tandemization of both elements critically influence HIV promoter strength. Mutagenesis affecting the U5 region of the HIV LTR was also carried out. The deletion of 26 nucleotides in the center or 3' one-third of U5 resulted in virus stocks with only modest reductions in replicative capacities. In contrast, deletion of 29 nucleotides positioned in the 5' third of U5 completely abolished replication. These results suggest that the 5' portion of the HIV U5 region contains heretofore unrecognized sequences/elements that are critical for infectivity; the results of preliminary experiments suggest that they may participate in RNA termination/polyadenylation.